High powered carbon dioxide lasers are commonly employed for industrial metal working operations such as welding and cutting. In such operations, the annular laser beam is reflected from a concave mirror and focussed so that the energy is concentrated at a relatively small spot. One of the difficulties encountered in such systems is that the depth of field obtained at the focal point of a mirror having a small f-number is very small. This makes the positioning of the work piece very critical, and also renders it difficult to cut thick pieces.
It has been suggested to overcome this difficulty by utilizing mirrors having a higher f-number to increase the effective depth of focus. However, the utilization of higher f-numbers causes additional problems since the work piece must be spaced a greater distance from the laser apparatus. This not only increases the floor area required for operating the laser, but requires stringent safety precautions to insure a safe operation thereof. Accordingly, it is desirable to provide a method of increasing the depth of focus of such concentrated laser beams without unduly enlarging the apparatus employed.